This application relates generally to fiber-optic communications. This application relates more specifically to methods and apparatuses for spectral grooming of optical signals.
The Internet and data communications are causing an explosion in the global demand for bandwidth. Fiber optic telecommunications systems are currently deploying a relatively new technology called dense wavelength division multiplexing (DWDM) to expand the capacity of new and existing optical fiber systems to help satisfy this demand. In DWDM, multiple wavelengths of light simultaneously transport information through a single optical fiber. Each wavelength operates as an individual channel carrying a stream of data. The carrying capacity of a fiber is multiplied by the number of DWDM channels used.
In all telecommunication networks, there is the need to connect individual channels (or circuits) to individual destination points, such as an end customer or to another network. Systems that perform these functions are called cross-connects. Additionally, there is the need to add or drop particular channels at an intermediate point. Systems that perform these functions are called add-drop multiplexers (ADMs). All of these networking functions are currently performed by electronicsxe2x80x94typically an electronic SONET/SDH system. However, SONET/SDH systems are designed to process only a single optical channel. Multi-wavelength systems currently require multiple SONET/SDH systems operating in parallel to process the many optical channels. This makes it difficult and expensive to scale DWDM networks using SONET/SDH technology.
The alternative is an all-optical network. Optical networks designed to operate at the wavelength level are commonly called xe2x80x9cwavelength routing networksxe2x80x9d or xe2x80x9coptical transport networksxe2x80x9d (OTN). In a wavelength routing network, the individual wavelengths in a DWDM fiber must be manageable. New types of photonic network elements operating at the wavelength level are required to perform the cross-connect, ADM and other network switching functions. Two of the primary functions are optical add-drop multiplexers (OADM) and wavelength-selective cross-connects (WSXC).
In optical networking applications, a given signal may encounter multiple optical components, some of which may have different responses than the others depending on the wavelength of the signal. When signals are multiplexed on a light stream carrying many individual wavelengths, such wavelength-dependent responses may cause undesirable variations between wavelength channels across the multiplexed signal. This variation may be particularly manifested as differences in the power spectrum as a function of the varying wavelengths. It is desirable to provide an efficient mechanism by which individual wavelength signals may be attenuated to correct the power spectrum to have the desired values across all wavelengths.
Embodiments of the invention are thus directed to a method and apparatus for spectral grooming of light having a plurality of spectral bands. According to embodiments of the invention, the apparatus may be provided as a variable wavelength attenuator. The light is received at an input port and encounters an optical train disposed between the input port and an output port. The optical train provides optical paths for routing the spectral bands and includes a dispersive element disposed to intercept light traveling from the input port. An attenuation mechanism is provided for independently attenuating the individual spectral bands. The attenuation mechanism has a plurality of configurable attenuation elements disposed so that each spectral band is attenuated in accordance with a state of one of the configurable attenuation elements.
In some embodiments, the variable wavelength attenuator may function simultaneously as a wavelength router. In such instances, the output port comprises a plurality of output ports and each spectral band is routed to one of the output ports depending on the states of the configurable attenuation elements.
In one set of embodiments, the configurable attenuation elements are provided as moveable micromirrors, with the state of each configurable attenuation element corresponding to a position of that attenuation element. The micromirrors may be moveable rotationally to achieve different tilt positions or may be moveable translationally. Also, the micromirrors may be configured to be moveable among a plurality of discrete positions or may be moveable through a continuum of positions. The micromirrors may be configured to direct the spectral bands to different portions of one or more common surfaces that have portions with varying reflectivities. The attenuation of individual spectral bands may be provided in part by such reflectivity variation.
The optical train used by the invention may be adapted in various ways, some of which provide single-pass configurations and others of which provide double-pass configurations. For example, the optical train may include a single lens with the dispersive element being a reflection grating that is used to separate the light into beams that correspond to the different spectral bands. In another embodiment, a transmissive grating is substituted for the reflection grating and a pair of lenses is used on either side of the grating. In still other embodiments, optical power and dispersion are combined in a single optical element that forms part of the optical train.